So I installed the GTS gauge cluster in my SR5 for my SR5 to GTS swap. I changed the underdash harness and rear body harness as well. The engine bay harness is still the SR5 harness. I'm just going to extend the alternator cable to accommodate for the alternator on the 4age. I still have my 4ac, I'm waiting on finding a decent 4age for the right price. All of the harnesses are plugged in together and the car starts, but I'm not getting any reading on my speedometer and tachometer. My headlights work fine and the e-brake light and check engine light works on the cluster. Any ideas why?
Thank you
Arvin

When I did my sr5-gts swap something similar happened to me, but it was a stupid fix. One of the gauge cluster plugs didn't seat all the way. Plugged it in till it clicked and it worked. Hopefully that is your only issue.

I would also use di-electric grease on the plug pins and contacts. That solved my no backlight issue.