I've had to repair the wiring on both of my 240s (both M47s) and I've replaced the switch on one of them (with help each time). It's got a large hex (maybe 22mm, don't remember), but even with the trans cross member off to let the trans drop a little, it's difficult to reach. It took a modified wrench to get the switch out. It takes long-handle needle nose pliers to get the connectors back on. This is on a lift with the trans dropped...still a huge pain in the ass.

An easier solution for you might be to reroute the wires to the dash and install a switch to activate the backup lights when needed. On both of my cars, the wiring insulation deteriorated to the point of the wires touching and/or breaking, leading to lights that don't work. The only reason I fixed mine was to pass the yearly state safety inspection.
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