It's often really hard to find service manuals for vintage or classic cars, and it seems your 1981 Pontiac Grand Prix is no exception!
However, if you're willing to pay for a copy of the service manual for your Grand Prix, then you can apparently download a copy of the manual for $13.00 here. (I have never used the site myself, though, so can't speak from experience.)
Alternatively, you can get a copy of the service manual for the 1981 Pontiac Grand Prix for download here, for £9.99 (UK pounds).
I can't seem to find a free version of the service manual online, but another good tip is that, if you type the exact repair you're hoping to carry out into YouTube, along with the year and model of your car, you can often find really good step-by-step video instructions for specific repairs that way.
Hope this helps a bit, and good luck!
