Although my 1989 GTA is very original, I am 3rd owner, it seems like the decals containing all the RPO codes are missing. How would I go about getting all the RPO codes for this car? Is that something PHS can help with?

 

PHS only goes up to 1986, they used to do newer, but GM wanted to take it over so they forced their hand.
You can get your Dealer Invoice through https://www.gm.com/heritage/archive, it will give you your options, the price etc.
It used to be free, but it was $50 the last time I looked.

 

As John stated, PHS was muscled out of the post-1987 documentation business by GM. The info you get from GM Heritage will only show what your invoice showed: as far as UPC codes go, the only thing you'll see will be the UPC codes for the optional equipment.

Your best bet is probably finding someone who has a car equipped the same as or similar to yours and "build" a UPC code list that way. Shouldn't be that hard to do, really. I can try to help you if you like.

 

https://www.compnine.com/vid.php

This is your best bet. They have an archive based on the VIN.

 

The problem in this case is the OP has a 1989. Compnine only has a small fraction of 1989s in their database so it's most likely not there, especially if it's not an exported one. Their coverage increases for the 1990 and newer years.