TriX has a great suggestion: Check the grounds. All of them!

I seem to remember swapping a motor years ago on an older Bronco and we didn't tighten the ground strap from the block to the firewall and I was having all sorts of weird happening. Turns out that's what it was. We had only put it on finger tight, so when we were looking around, it looked like all was well. Only after putting a wrench on it did the ground actually ground. Coincidentally, I just had another ground issue that was making my lights wonky. Grounding is very important in the electrical loop!

Welcome and good luck!

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