There are fuses underneath the dashboard on the drivers side in xj models.
If it is an xj the heater blower is bolted into the passenger side of the bulkhead. It's a pretty simple job to remove it, open the bonnet, move a few things out of the way like the evac canister, pull off the connector, undo three nuts and pull the whole thing out. TBH they're pretty reliable, it's more likley an electrical fault elsewere.