You can connect a power source directly to the motor leads. I have a small plastic battery 'box' that holds 4 AAs with alligator clips that I use to test motors and circuits. It'll give you 5-6 VDC which is good to test most things we use in 1/16. I pieced this together from spares, but Radio Shack sells a variety of AA batt holders.

I'd check connections too. After that, wires. My HL StuG elevation quit after a day or two, it was a bad wire. If you don't have one, a multimeter is a needed tool for these tanks for a variety of applications, but in this case continuity checks are essential.

I pulled apart a 10 year old Tamiya flash unit last week that quit on me. I assumed it was the xenon tube or trigger coil since it was getting power from the MFU. I ordered replacement parts, but couldn't get it to work. Decided to test the trigger wire and sure enough, it had no continuity. It could have saved me $15 and a lot of time if I just tested from the start, but live and learn. Besides now I have parts to make 5 flash units, which @ $60 a piece, just being able to make the one I'll need for my next build pays off.