A shot in the dark

My method, if it were my project and I had no docs: Power it up "normally", and either trace (from the power supply inputs) or find the GND and V+ traces on the board using a VOM.

Power it down, and while leaving all the pieces hooked together, find a component on the board that taps the V+ rail and clip one lead there, then use the other end to find the closest-to-zero ohm pin on the connector at the printer end. Repeat for GND. Repeat for any other power there might be. Once you've resistively located (or think you have...) the V+ and GND traces on the board, trace back from the printer to be certain you've got a connection like you think you do. (I'd expect to find multiple lines of the printer side that go to GND)

Power it back up, and at the printer end, note the voltage(s) of the hot(s) versus GND, and there you are.

Apply appropriate power to the printer at those pins and see if you get the expected behavior.

If you're talking about the type of thing I'm envisioning for this printer (more or less a snap-out "printhead and paper feed" unit), I'd assume (at least until finding out otherwise) that with the board removed, the printer module is completely "stupid" - Meaning that I'd expect that you'll have to go in at the "Step this motor twice and activate this dot-maker now step this motor once and activate this other dot-maker and that other dot-maker..." level of control. Because of this, it *MAY* be easier to interface to the board rather than the printer - there may be on-board capability to handle the translation from something handy like ASCII to the actual move/activate sequences needed to make the glyph. I've done a similar "frankenstein" to drive a wireless tilt/pan camera mount - turned out to be a lot easier to just interface to the salvaged board and its existing drivers than try to do all the driving myself.

Good luck!

that brings to mind the scarry thought of 'winprinters' (like winmodems) where instead of an onboard controller, everything is controlled via software drivers on the computer.... ick..

dan