Ok I know this has been covered many times but I've tried just about everything I can think of and it still doesn't work. The cruise control switch on the dash operates and illuminates but the system has never worked since I bought it about three years ago. I wasn't too bothered until now but as we are planning a trip down to Corfu in a few weeks I thought I'd get it working if I could. I started with the hoses, they all seemed fine but I cut back all the ends anyway hoping it would be as simple as that as it seems to be the most common issue but this made no difference, I noticed that the switch on the brake pedal wasn't sitting square and would'nt locate properly as one of the clips had broken off so I got a new one and fitted it, no good so next I got a used replacement vacuum pump and fitted that, still no difference, next I found and changed the relay behind the glove box, no good so I got myself a used ecu and fitted that and it still doesn't work. I've checked the fuse and that's fine as the other things on the fuse work fine, I've read some stuff on here about something related to the steering wheel and the switches but I think this also affected the horn, my horn is fine. Can anyone suggest anything else? 98 auto 300tdi by the way.

A leaking vac hose is the most common issue I wool replace the lot if I was u.
The throttle actuator diaphragm can also leak and I assume you have checked is linkage, also that your speedo is ok as the speed sensor can fail which the CC shares.
The rotary coupling below the steering wheel could be faulty, it is multi circuit the one used for CC could be faulty but very unlikely if other citcuits are ok.
If manual gearbox there is also a dump switch on the clutch pedal. Unfortunately the insitu electrical circuits can be tested with Testbook or similar.

Although I'm convinced the hoses are ok I'm now thinking that I'll get a few metres of new hose and replace them just in case. I've checked the throttle actuator diaphragm which is ok and the linkage is fine, it's an auto so no clutch pedal switch, I'll try new hoses and let you know how it goes.

After a few years of cutting the split ends off the vacuum hose in the engine bay I finally bit the bullet and went for something similar to this stuff, the CC works every time and wouldn't be without it now.
http://www.ebay.co.uk/itm/311007432657?_trksid=p2060353.m1438.l2649&var=610298486276&ssPageName=STRK:MEBIDX:IT

Looks good thanks, I've ordered it, I'll let you know how I get on.

Ok, time for an update, fitted all new hoses and its still not working, think I'm giving up on it now. Thanks to those who tried to help.

Before you give up you may want to have a look at the cruise control brake switch. I believe it can be adjusted.

In case you'll decide to not give up though as you'vw already worked so much some ideas:
1. can you see the engine malfunction warning on ignition self test?...cos if you cant it's possible to be some engine malfunction code stored which you dont know about it(even if the engine runs fine) then the ECU energises the SS disable relay
2. the fact that the horns are working doesnt rule out the rotary coupler cos there are different circuits for each system wihin it, so if you still want to continue download this document https://www.google.ro/url?sa=t&rct=...zfiQxFyNy-zYrnYAxycAfQ&bvm=bv.105039540,d.bGg cos it has the diagrams and connector views and i'll tell you where to measure with multimeter to check continuity cos there are 3 circuits within the rotary coupler involved CC functioning

You do have your vacuum hose connected to the correct port (black) on the vac pump don't you.
Before u start removing the steering wheel and inspecting the rotary coupling

To check continuity you have to get only to the connectors by removing the nacelle(1 minute job) might not be necessary to remove the steering wheel ... though i might be wrong on this one

All hoses are fitted correctly, I don't have a multimeter as I wouldn't have a clue how to use one, I'm no good with electronics, anyone nearby fancy having a look for me, have to be in a few weeks time though as I'm of to corfu in it tomorrow?

I had a similar problem, however after trying the electrics I went out and bought a hand operated vacuum pump, I connected it to the vac pump under battery and watched to see if the vacuum was sucking in the actuator on the wing, it did but very poor, like you I had changed the pump also, however I wasn't convinced so searched and bought another changed it, it worked perfect, then I changed all the hose every in with petrol pipe checked vacuum again even better and quicker. Ok so now time to try all electrics again, I replaced the relay and could hear it clicking in so thought all ok, took it for a test run nothing. I removed the switch and tested it it was a little iffy so again changed that, now I could here it clicking the relay on and off so thought I'd cracked it. Took it out again no luck, couldn't trace it at all changed everything,
Then by a bit of luck while out driving I thought I would try it again, this time got upto speed and pressed the switch nothing first time so tried again nothing again, so while just about to give it up as a bad job I started messing with the steering column switches and it started working, in fact to me aback but it worked, I received the column switches and changed them with some from wreckers cleaned up contacts and as been perfect since, haven't a clue if this was anything to do with it, but it's working.
I think the switch I messed with was the set switch, sorry can't shine any more light. But don't give in you done the hard bit.
Hope this helps a bit.
Grez