I've never had the need to check the control on mine, but you should be able to see (or at least feel) the wires from behind the dash and push the connector on tight. If there'* other stuff in the way, you can remove the plastic dash trim panel and then maybe remove the mounting screws and pull it out to access behind it, but I'm not sure.
On my '92 and '93, the dash trim snaps off - there are a few metal clips that apply pressure to hold it in place. Tool companies make a special trim removal tool and you can use one of those, but I used my fingers and slight (careful) pressure with a flat-blade screwdriver.
I doubt that the potentiometer (adjustable resistor) would have failed completely, even if it was used quite a bit, so I hope it'* just a bad connection.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles