I can't start my car in the cold... help me!!?

I have a 1996 Ford Probe GT..it has a customized Mazda DOHC MFI 24 Valve 2.5 Litre V6 with a six speed manual transmission.. Great car, runs great... i've had the car sitting for about three weeks while i was working on my Hyundai Tiburon project, and now it won't start.. the battery doesn't seem to be the problem... i've charged it and tried to jump it... the starter turns over and sounds healthy, my lights work, radio works.. i'm installing a dipstick oil heater, because i've heard that can help a cold start.. i haven't installed that yet though, the part is on it's way. will that do it or what else should i do? this is a great car.. it's my baby.. body kit, rims, system, air intake, the works.. this car has never given me any problems, and everything under the hood is performance, i don't mind spending the money to fix it, but if there's an inexpensive way out of this that would be nice, and i'd prefer to fix it myself.. please help..