If you can't hear the motor whining, you will need to go through the system connetor by connector with a voltmeter and / or continuity tester.
I would guess if you are getting a clicking sound the relay is throwing, which means the conrol circuits to the remotes are OK, as is the relay coil so that limits it to one of four things:
1) relay contacts are burnt out.
2) Fuse supplying PT motor has gone
3) loose connection.
4) Motor is gubbed.
Based on your previous symptoms I would be swaying towards 1 or 3, with a possibility of 4, but read on....
Firstly, find the place the motor plugs into the rest of the wiring (follow the wire). Disconnect, and put a voltmeter across the terminals. (set to read at least 12V if it's not auto ranging)
If it's 2 wire either test lead in either pin any way round will do, if it's 3 wire you may need to take a couple of guesses as to which wire is earth, although you may be able to trace that back through the loom)
If it's 2 wire, pressing the up & down switches should throw the relay (you'll hear it click) and you should get either +12 or -12V on the meter. (should swap as you go up & down, 0V for neither)
If it's a 3 wire, the relay will power up one or t'other of the coloured wires (blue / green from memory) when the meter is across them & black (earth) so you may need to test a couple of combos of pins with the switch in either up or down position.
From memory the motor should stop actually spinning around 8-ish V, so if you get a positive volt reading , but nothing near 12, chances are either a loose contact further up the wiring loom or dodgy relay contacts. To test the contacts, follow the loom & chek at every connection you find
If you have a full 12V in both directions, your motor is gubbed. There is a high chance either your motor brushes are so worn that they aren't now touching the commutator, or as I had on mine, a broken brush spring so no contact whatsoever.
Let us know what happens with the voltage test, as dismantling the motor can get fiddly, so if you get 12V at the socket, I'll try & talk you in! (if that's what happens, post back with the serial number as that;s awfully close to the demise of the clamshell, and so I just want to make sure I;m talking about the right engine!
The plus side is that brush sets are a LOT cheper than a new motor if you don't mind taking it to bits!