While I do not have direct familiarity with either of your skis, I do have electrical schematics for all 96s that is attached. You can also find a schematic for your 95 XP in the service manual download from the manuals link on the forum. The first thing I would check are the 5A fuses located on the MPEM modules on the skis. Since both skis run but no gauges work, the gauges might not be getting 12V power from the MPEM. The MPEM supplies 12V to the oil-fuel gauge on either ski and that gauge passes the 12V on the the other gauges. If the fuses are good you could check the 12V at the Red-Purple and Tan-Black lines at the oil-fuel gauges with a voltmeter. I would think your 96 GTI would power up the guages for about 30 seconds when you attach the key without even starting it.
I'm sure others with more direct knowledge of your 2 skis will offer other troubleshooting tips, but that is where I would start based on your description. Good luck.
