Had the same problem on the passenger side of my '98 XEDi.
Strip the regulator and gear out, if it's the cable snapped in the middle, you'll need a new piece of cable.
Mountain bike gear or brake cable is about the right size, and designed to resist salt/mud etc.
If you don't have a gash bike sitting in the garage just waiting for this, you can buy a cable at a bike shop. The cable comes with one end already with a barrel attached.
You need to undo the regulator motor and undo the other end of the cable from its housing. Feed the new cable in, with the cut end into the regulator motor, and adjust the cable to the right length. I used a 3 Amp electrical connector block with a small washer, but had to cut a small area of the plastic (nylon?) capstan in the regulator to get the parts to fit.
One of the cables has to be wound around the nylon capstan a few times, with the other cable being fully taught. I used a BNC crimping tool to cut ther cable, and nipped up one of the 3A connector screws to allow exact cable measurement required, before fully securing with both screws.
My contraption worked for a couple of days and then failed. I had to rewind the cables, redo the connectors and used solder to fully secure them, and added an epoxy resin to secure the connector block, washer and cable into the capstan. Its not coming loose now...
When the original cable had broken, it had wound itself inside the motor, and caused the metal to bend outwards slightly. That had allowed the new wire to come loose off the capstan. So make sure the metal is flat. A few taps with a small hammer straightened it for me, and the windows been working OK for the last few weeks.
Cost me nothing but time, because I had the tools and kit lying about, I'd reckon about £3.50 for the gear cable from a specialist or smaller bike shop. and about 99p for 3A terminal block.
I've hopefully added a picture of the layout of the regulator FYI.
Good luck!