David,
I've had the dubious pleasure of fixing the Bachmann trams - I currently have 2 Christmas ones that light up but don't move when you power the track.
When you remove the motor bogie, I recall the wiring is visible on the underside of the floor. It is easy to spot that the head/tail lamps have a diode so they only light when power is in the 'right' direction.
The motors are very weak. They overheat and the brush gear area melts. I have replaced the motors - Bachmann's HO scale SD45 seems to have the same one, but whether it is designed for 14V-20V I do not know. The recommended solution is to toss the power bogie and replace it with a USA Trains 45T diesel bogie, or a Piko (I have a tram loco with a piko power truck and it seems to still be running.)
You might find this thread useful when the time comes:
Large Scale Central - Advanced Forum Detail Topic - Re-motoring a Bachmann 4-wheel Trolley
Since then I have found that the MABUCHI FK-130SH-09450 motor, which is readily available, is the same footprint so I bought a couple on eBay for $4 and I will be using them in future.