Sorry guys, I'm a Ham Radio guy and I had a stainless steel whip that I just cut to length and inserted in a threaded fitting that worked.
On the other hand, I have seen in auto parts stores, a "universal fit" car antenna that had 3 or 4 different threaded studs and a metal antenna whip. That would work for the Promaster.
If you unscrew the antenna, and take it to a parts store like Pep Boys, Autozone, etc. I'm sure they would have a replacement antenna that you could buy and then cut to length. Just make sure you buy a metal antenna, not one of those rubber or fiberglass ones. The metal ones are usually stainless steel and can be cut. The rubber or fiberglass ones usually have a coil of wire inside, and don't work right when cut.
Next time I'm in a parts store, I'll look for a specific model, but I'm sure that you will find one in your local parts store.
Ed