Hi Labman,
As Owen said, you have to remove the gas tank to get at the nuts for the knee pads.
As timing would have it, I am in the process of re-upholstering my knee pads. I've taken a couple of photos to show the locations of the nuts Owen was talking about. In the photos, they are where the yellow circles are, situated right in the bend of the sheet metal of the body. My car is Right-Hand Drive so, the photos may be mirror images of what yours looks like. The larger knee pad has three studs/bolts holding it on, the smaller one has two.
Taking the gas tank out is straightforward: Disconnect the fuel line and the two wires. The two bolts easily visible at the ends of the tank straps are all that is holding it in. Remove the bolts and straps. The straps and tank will then come right out. If you have a lot of fuel in the tank, you may want to drain it off first leaving the fuel line connected at the tank and disconnecting the fuel line in the engine compartment, preferably the lowest-most point in the line. The tank should then drain via suction.
With the tank out, all the nuts for the knee pad are easily accessible. Altho for me, I had to remove one of the windshield wiper motors to get to one of the nuts. In the first photo, the left-most yellow circle indicates that the nut for the knee pad is behind the wiper motor mount. To do this, you'll have to disconnect the motor from the linkage (watch for small washers that will come off, too), undo the three bolts and disconnect the wiring harness. Maybe a good idea to disconnect the battery first, too.
Cheers,
-bob
