I want to put 85-later style tail lights on my 84 Trans Am. Will they go right on or do I need to snag the tail light harness from the later car? TIA.

 

I'd wait for a few more responses to be sure, but I think they all fit, some require moving the lock further out I think. You say 85 and later, but which ones? I made this little guide for those who want to compare styles.

 

I was gonna do the Formula style ones.

 

If you want to do the Formula ones (which I don't think anyone in their right mind would do, but thats my opinion) you need a new wiring harness. If you did the TA ones you don't.
The base model taillights have all kinds of little bulbs and connectors that the normal TA taillights do not.

 

So if I went with the 85-90 trans Am style ones, they would just plug into my existing harness?

 

yep