I literally installed a new head unit in my MK4 today. Pioneer DEH-S320BT.
It was fairly easy because it came with the correct harness for the MK4.
The 2 things that took the longest were shifting the cables into little crevices so I could push the unit into place and I had to come to the realization that I had to swap the red and yellow cables. Until I did the unit would stay on after the car was turned off and when it finally did turn off it would lose the clock and settings every time I turned the car on. Simply swapping those two wires fixed the issue.
https://www.amazon.de/gp/aw/d/B07X6KG3W1/ref=ox_sc_act_image_3?smid=A3JWKAKR8XB7XF&psc=1
https://www.amazon.de/-/en/gp/aw/d/B018KH7N18?psc=1&ref=ppx_pop_mob_b_asin_title
Good luck. I was using a tape adapter for the last two years. Finally I said I'm done and bought the parts.