Both mine came from ebay, I brought the black one first, its made by Volvo Penta and the quality is obvious. I thought if one is good then two must be better, the second grey one was similar to all the generic ones you see on ebay like this
https://www.ebay.co.uk/p/Seaflo-in-Line-Bilge-Air-Blower-130cfm-Boat-Black-Ventilation-12v-2-5amp-L3l1/676977437?iid=322930682984&_trkparms=aid=222007&algo=SIM.MBE&ao=2&asc=49925&meid=6a43620189884b4b9381f2c6f87b2df2&pid=100623&rk=4&rkt=4&sd=282400224490&itm=322930682984&_trksid=p2047675.c100623.m-1
Both fans claim similar flow rates but the Volvo one is more powerful than the grey one and while having two in series gives a useful increase in flow I dont think you can just add the flows together and expect that as your output.
One other thing, I dont have a standard Series 3 lower dash / heater box, I made my own which allows a lot more air to flow, it has seven large outlets five of which can be throttled plus another two smaller outlets for side window demist.
Both units had the same type of flange fitting so I guess its some kind of marine standard. I had to make an adapter to allow the fan to directly mount to the heater unit then I just coupled it all up with flexible ducting and ran the inlet pipe to the front for a fresh air supply.
