I have a 1998 Porsche Boxster, and the key that I have has two buttons on it; one for each of the trunks (front and back). I am wondering if I could have a key programmed with a button for the doors, but I don't know if the car had that feature (I purchased it used). Was this car available with keyless entry for the trunks but not the doors?
If your vehicle already has a fob to open the trunk lids it clearly has a receiver, and if has power door locks, there's no reason it should not also have keyless entry unless the manufacturer was exceptionally lazy. Lots of medium to high brands had keyless entry around 1998, including VW.
Shop around for aftermarket clones of the factory key/fobs with the correct buttons. You can always get a legit one from the dealership as well, but it's going to hurt. Probably a couple hundred for the fob and another hundred to program it (which is often a painfully easy procedure you can do yourself and might take 5 minutes). Given it's a Porsche that estimate could be way low.
Worst case, if there are power door locks then it's pretty trivial to wire in a complete aftermarket no-alarm keyless entry system that would replace the factory fob and receiver. Any good car stereo place should be able to install something like that and those parts are pretty affordable these days. This could easily be your cheapest option.