I'll proably do it myself because I dunno, it seems when I take stuff like this into "professionals" things come back messed up (other electrical not working etc.) And I like the challege, or um, I like to challenge friends too!

Good point. I certainly have heard a lot of horror stories about professional installs myself.

Installing an amp isn't much harder than installing the speakers or the stereo, except for getting the wires to run nicely into the trunk. I've done an amp in an '88 GTI, which has a lot of similarities to that '91 Jetta. The hard part is just knowing what bits to remove to get at the wiring channels.

Just remember to use a decent wire gauge for the power and ground wires, and run the power directly to the battery (through a proper fuse block and connector or else you run the risk of fire).

Tips on installation can be found linked from here. And if you're going to be installing it yourself, you might want to order it from crutchfield because they'll also make sure to sell you all the necessary bits along with a good booklet that's got instructions on how to do everything right.