Just did this on my '82 242Ti... Wanted a later model window with the 3rd brake light and a properly functioning rear defogger (mine worked, but was slower than mo-lasses). Really wasn't too hard a job. About the slowest part of the job was getting the old seal released from the lip on the body. After doing that, it really wasn't bad. It does help to have someone pressing on the glass when you're installing the new piece of glass, since then the seal will be more willing to engage the lip.

For wiring up the 3rd brake light, I ran a wire under the carpet along the driver's side of the body to the brake pedal switch, bypassing the bulb integrity sensor. Remove the fuse for the brake lights (on a pre-'85 240, it's usually fuse #6). The wire to tap into is the blue/red wire on the switch. The ground wire on the light should be attached to one of the screws used to attach the duct under the rear window to the body itself.

After wiring up the light, and testing to see if it works, start putting things back together, such as the C-pillar covers, the seatbelt retractor covers, the hatshelf, etc... Then enjoy having a 3rd brake light on an earlier 240 or a 140. Oh, btw, if your car is a turbo or a GT, and you want black trim, just remove the chrome strip from the rubber seal and clean the rubber up real well (or just get a rear window from a '91 or newer 244).

-J