Hi,
An old roommate rewired my furnace (DC90) so that when the power to the furnace is on the blower motor runs continuously whether thermostat is turned up or not. Any help is greatly appreciated.
Mike

Either thermostat not disconnecting(bad thermostat). Relay or contactor fused closed,
Or short in wiring.
Disconnecting 1 wire at a time on thermostat may quickly isolate.

We don't know what your room mate did to rewire the furnace to make the fan run continuously, he may have bypassed the fan relay or put a jumper from the "G" terminal to the "R" terminal, if you have a schematic wiring diagram you may be able to compare it to the existing wiring and make a determination of what has been changed, Good luck, Mike

Agree with acetc, Roommate misswired accidentally, or intentionally(To have cooling, without replacing defective part).
Red Is common at thermostat, when it switches to Green(Fan),Fan Kicks on.