I know you can buy an additional under the seat heater core that you can hook up to your heater lines. It probably wouldn't be too hard to make out of another heater core. Put it in a metal box with a small electric fan and put holes in the metal box. Changing the thermostat isn't going to fix the problem IMHO. Your heater core is probably corroded.
Also, check the tubes coming from the heater housing going up to the vents. They are probably cracked and ripped. They are just a paper like fiber around a coiled wire and they are old and deteriorated.
