Hi
I just fixed my rover 25 heater speed. take the glove box out, 2 screws, and you can see rester pack, this comes out with 2 screws too.
I bought a ceremic resistor pack off ebay, it came with intruction on how to fit and how to remove glove box etc. all you need is a screw driver and soldering iron. it cost about £5 for the resitors. only take 5 minutes.
window heater, is it just one line that does not work? if so any breaks can be very hard to spot. You need to go along with a test lamp (with a very sharp point), make sure heater is on, connect one end to the heater connection on the boot, and go along sticking the pint in to the line until the lamp does not light up, you have then found your break. then use the paint that you mentioned.
Not sure about the speedo problem, i can only this of a loose electrical connection.
hope this help
Adi
May 2009