I'm doing the wiring today for my alcohol injection (using a relay to control power on only when the key is on), and I see these two inviting looking 30 amp circuits labeled
30 A Windscreen heater left
30 A Windscreen heater right

I wasn't aware my 2002 XJR had a windscreen heater...
If it does not, I'll just take my main power here,
if it does, how does one activate the windscreen heaters?

 

They are for the windshield heaters.

Not many cars came to the US with heated windshields. Probably only the ones that had the pollen filter.

 

The circuit is activated by the switch to the left of the rear defog switch.

That presumes that the switch and harness was actually installed,

The heaters are timed, so it may be switch->bcm->relay

The electrical diagrams will tell you more.

 

The heater switch is labelled "F" for front, and it puts on the heat for approximately 10 minutes, so it's not an on/off thing.

 

plums is spot on, no use for the application. To add the heated screen you need the link lead across the bulkhead and a heated screen. Easy install, many of the later UK cars are equipped with it, in freezing weather it's a superb function.

 

Which fuse panel is this?

 

However the wires wreak havoc on the homelink garage door opener and phone based GPS.

They still work, but the signals are very tempermental.

Come to think of it, where is the onboard nav GPS antenna? In the backlight? Maybe that can be put to good use with a passive repeater. Passive repeaters can be built by trimming a piece of coax to tune it to the required frequency.

@waterdragon,

You might still get some use out of the circuit because the input to the power pole of the relays might be connected. In that case, you can choose to tap into it, or tap into the output and use the relay to control the circuit some other way. If you choose the latter, make sure that you isolate the control input from the BCM.

 

I ended up using a relay:
taking power off the big wire going to the fuse boxes (using an inline fuse) and then controlling the relay with the key on only rear cigar lighter because I found very easy access to run a wire through the firewall where the hood release cables go through, and super easy to run a wire to the rear cigar lighter which made for an easy and very clean installation

Thank you everyone for all your help