heater a/c

1990 cherokee blows defrost only

You have a vacuum leak. There are a zillion posts already on this/ Try the search feature.

Not that it helps but my 99 only blew on defrost and one day it fixed itself

The damper doors are operated by a vacuum valve. A tiny vacuum leak in the vacuum line near the firewall and AC/Heat hoses is the common location of failure, but a larger vacuum leak under the hood anywhere, or a bad diaphragm in the valve can also cause it. More detail in old threads here, try searching "vacuum leak blower", or "vacuum leak defrost" for starters