Drivers side, just above the rear corner of the pan, about one inch behind the shift cable bracket.
On my '73 it's a two wire connector, pink (?) & black (ground) but if I remember correctly, the solenoid itself grounds through the trans case and the hot (pink) is the only connector lug on the trans connection. You should be able to test with a test light plugged into both connections of the wire plug. Mine has electrical operation, but still won't downshift except occassionally. It's old: 100,000 miles. On the list to fix.
Might want to check the vacuum connection to the vacuum modulator on the opposite side & farther forward. It has a rubber vac line a couple inches long connected to a steel pipe going up to the engine bay next to the trans dipstick and then rubber again to the manifold tee. This line can fill up with crap and block vacuum, as well as split open.
The electric solenoid is for full throttle (pedal to the metal) down shifts. The vac modulator is for 'other than that' down shifts, as well as upshift points. It works by engine load, as determined by vacuum. The vac modulator can be adjusted a few hundred RPM using a teeny (or it might be tiny--I don't remember exactly which size) allen wrench to turn the adjuster inside the vac line connection tube nipple on the modulator itself. It will turn only 180* or so, but if it's the original modulator, that probably won't make any difference. Can't remember which way does what, but I think in (clockwise) raises the shift point. A brand new one from NAPA only costs about $7 and is a half hour job to replace. Including beer break.
The actual solenoid is in the pan. Like Jphil said- check for 12v at the tranny before you get too excited. Key on, meter or test light between the case connector and ground, mash the gas- light should come on. If not, keep digging OUTSIDE the tranny. If it comes on, check resistance on the trans connection to ground. Should have a circuit. If not- solenoid has died. Easy to replace- drop tranny pan, 2 bolts holding solenoid on, new filter and pan gasket, refill, and boogie!