So I swapped out the power supply I hadn't been able to get to work for one my brother had lying around, and voilá -- success! My car bulb lit up, as did one of my stepper motor driver boards. The other board has a short, it lit up briefly and then went out, after which the 12V line was dead until I unplugged the power supply and plugged it back in. However, the 5V and 3.3V lines were fine and under load from the 10 ohm 7W resistors put in -- which got quite hot in the process, as my fingers can attest to after trying to remove one:)
Pictures of the board with the short:
On the down side, I found that the acrylics should be about 1/4" thick (which is 6mm, mine are 3mm). To not waste the drilling I just did, I'll try to glue two pieces together.