1991 VW Passat speedo help

My girlfriend has a 91 passat with an electronic speedo. A while back it would work intermittently and now it has just plain quit. Anybody got any ideas as to what this might be? Also it's been notoriously bad for overheating. The cambelt snapped a long while ago and the head was entirely rebuilt by a shady company that has now gone out of business. They broke the heater core and it was extremely expensive to get fixed. Around 4 grand expensive! Anyways, I'm fairly technically savvy and I've come to my wits end with all of these problems. I think maybe the speedo may have something to do with the sender off the tranny but I haven't got the volkswagen service manual on the car and have no way to test it. As for the overheating, The water pump, the thermostat and the heater core have been replaced. Recently it overheated to the point where it could no longer be driven and I found that the seal on the bottom of the overflow tank cap was pinched. Fixed that and it still seems to be losing coolant. Anybody got any ideas before I go crazy? The damn thing's only got 150,000K on it and it shouldn't be doing this. Thanks in advance