Have a look at the following post, assuming your blower motor is working fine (electrically) at all the speeds through the resistors, and the high blower relay, then you should go through the vacuum and air flow troubleshooting steps to determine what might be wrong.
Note, there's also the possibility that you have a lot of build up dirt on the the heater core, and/or evaporator core that is hindering air flow from blower motor from getting to the inside HVAC box. Also, you could have a lot of air leaking from dried out or missing foam seals around the plastic ductwork behind the dash. I have this issue on mine with the AC on, I can reach my hand behind the radio pod and feel a lot of air leaking that should be coming out the dash vents. I have to get that old foam sealing replaced one of these days.
Credit to
Ribbedroof for posting the service manual pages to help troubleshoot.
Here are the pertinent pages from the 88 shop manual, which is pretty much the same system as all G bodies OTHER THAN the automatic A/C system found in higher option carlines (I think Buick and Olds...never seen one in real life)