Hmmm, no photos, so I'll guess your '76 looks a lot like my 75'. If you didn't cut the control panel wires off right at the interior skin, you should be able to hook up a switch to the old pump wire. There would also have been a "hot" 12 volt
wire in that bundle. So, hot 12 volts to switch, pump wire from switch to pump, ground the other pump wire.
The Converter and pump are both under the sink, right? If your wires are all cut short, you could run a wire from the fuse block to a switch, back to the pump, then ground the other pump wire to the frame. This is a DC motor so it's important to get the polarity of the wires right or the motor will run backwards.
You could put the switch in a convenient place under the sink, but visible, so you wouldn't have to run wires up to the upper cabinet.