OG mk1 golf stereo - how to modify for aux input

Depends on the exact make and model of ye olde stereo I think.
Mine had a separate amp IC for the tape deck so it was all a bit easier. There were effectively two different devices - the radio and the tape deck - and the amps were switched by an internal switch operated when you pushed a tape in the slot.

The internal switch (behind the door where you would push the tape in) has been re-routed out to the dash in a completely sketchy looking, non-OEM switch and I just had to put a 3.5mm TRS type female connector in place of the tape input.

You'll need a tiny soldering iron tip and a pin-out of the ICs in the stereo. If there are any blank switches on the dash, you could modify an OEM switch to select between the two inputs. That's what I should have done.
Also, I should have routed the 3.5mm jack for the iPod to a better spot, probably under the dash at the driver's side. I presented it in the cubby hole between the front seats and it's not convenient for either driver or passenger to operate. Think about power, too.

This would be a great place to put some pictures but I'm at work and they frown on having access to photo hosting sites.
I'll try and remember to edit this later with a couple of pics.