Having just completed my first upgrade, a Premium GP9, I come away with this.
1)Go slowly. Stare at the PS1 wires, before you tke it apart, as you take it apart, etc. If they have connector boards at the front and rear, see if you can reuse them to help simplify wiring. On mine, these boards connected to inserts that held the number board lights. Sadly it was 4 days in, after much cutting of wires that I tried the original boards
2) Mine had dual lights for the number boards in PS1.The kit didn't come with that. I ordered 2 sets of cab lights (the one with 2 white bulbs). Call midgge and give her the part id from the upgrade kit.
3) the marker light wires are too thick. I stripped the shrink wrap off 1 wire. This kept the 2 wires separate from each other. Then i slipped shrink wrap over the bare wire and still covered wire. This allowed me to make it thin enough to fit in the hole. Note the old harness was built this way by MTH. The upgrade and replacement marker lights are made incorrectly (IMO)
4) I went to a BCR to save the space used by the battery. This also allowed me to forgo a charge port. This wa im[portant as the GP9 is quite busy on the bottom.
5) Stop at any point and post questons.