One of the more common and annoying, though not critical, failures in the 924 is the lack of working rear defrost/defog. Fortunately the system, though electrical and easily susceptible to gremlins, is simple and easy to diagnose and fix. Power is supplied via a relay back to the left-hand side of the hatch, then grounds on the other side of the hatch. Power is transmitted on both sides through the hatch struts themselves! Since the right-hand side (RHS) strut grounds to the body, the left and right struts are not interchangeable. This can be one of the most common failures - swapping the struts will cause the fuse to blow every time the defog is turned on.
First place to check is therefore the struts, for correct assembly. If a
strut with a grounded end is fitted, it should be on the RHS, with the
ground (ball end) connected to the body. Both struts should be "inverted" - the body of the strut at the top connecting to the hatch, the rod at the bottom connecting to the car body.
Wires should connect at each side of the hatch; from the car body to the connectors on the bottom of the struts, and from the connectors at the tops of the struts to the hatch defrost grid. As mentioned, the LHS connector will supply power; with ignition on, rear defog switch on, and relay working/fuse intact, you should be able to measure +12V on this end. If not, there's a problem with the power supply, switch, relay, or fuse. We'll come back to that.
Assuming you have +12V there, check the other wire on the RHS of the hatch for ground. It should be a good ground. Make sure both wires show no sign of burnt insulation; if so, replace them. The ground wire connects to a spade type terminal on the right rear wheel arch, easily visible after peeling back the carpet on that side of the trunk area. Make sure it's a good ground connection.
Likewise, check connections through the hatch; from between the two connection points on each side of the hatch, there should be continuity with some degree of resistance, somewhere between 1 and 8 Ohms. If not, check the struts themselves, as well as the small wires that connect the struts to the defrost grid, not to mention the defrost grid itself. Sometimes a poor or corroded connector will cause a problem here, or a broken/burnt wire. Keep in mind that the defog uses a LOT of current if you choose to replace any of these wires; use at least 12-gauge wire and heavy connectors. Anything less can run a risk of fire.
A lack of continuity in the grid is unlikely; however it is quite possible that one or more lines in the grid are damaged and no longer conduct. This is very easy to spot when the rest of the grid is working; there will be a line of fog remaining all the way across the hatch. Very annoying but easy to fix; rear defog repair kits can be purchased at most auto repair stores, and are just a transfer you apply on top of the existing grid to restore the circuit. If you have a tinted hatch, that will be a problem however; the tint must be removed to get to the defog grid.
This should resolve any connection issues back at the hatch, so back to
the switch/relay. The fuse for the defog in some cars is on the top of the relay.
Naturally check connections for corrosion in addition to checking function. The defog
switch can be popped out behind the dash by reaching up from underneath; check for
function and power supply from ignition. Refer to wiring diagrams in the Haynes or factory
manuals as needed. For more information on testing and diagnosis of electrical systems in
general, refer to: http://www.924.org/techsection/elect_diag.htm