Since no one else has piped up, I'll give it a try. I know nothing about the Lucas switch, but the relay is easy.
I think a 40 amp relay will be more than sufficient. 70 amp ones are available if you really want overkill on the amp rating. Run the 2 yellow wires to one side of the contact. Install a small jumper from that terminal to the battery terminal on the Lucas switch. Attach both GY/Y, 1 R/LG and 1 BK/LG to the other side of the contact. Run a wire from the run terminal of the Lucas switch to the relay coil (terminal 86). Wire the other side of the relay coil to ground. Attach the W/PK wire to the start terminal of the Lucas switch. Getting a DPST relay might make it a little easy because you could install 1 Y on each contact input and then 2 wires on each output. Instead of splicing 4 wires to one output terminal on a SPST relay. Also, verify that the run terminal of the Lucas switch has continuity to the battery terminal of the switch when the switch is in start, as well as when it is in run. If the switch doesn't have continuity between run and battery in the start position, you'll need to install another wire between the switch and the relay, along with a diode.
Thanks for taking the time to post a response. Here's a picture of the ignition switch. Sorry it's blurry.
I've never wired an ignition switch before, so I don't if they all have these same terminals, but I can't tell what's what. The terminals appear to be numbered 1 through 4. Do the numbers refer to each key position?
I have no idea what the numbers indicate. Use an Ohm meter. With the switch off check and see which terminals have continuity between them. You may not have continuity between any of them with the switch off. Turn the switch to the accessory position. You should have continuity between the battery terminal and the accessory terminal. Turn the switch to run. You should have continuity between the battery terminal and the ignition (or run) terminal and between the battery and accessory terminal. Turn the switch to start. You should have continuity between the battery terminal and the start terminal; between the battery terminal and the ignition terminal; and possibly between the battery terminal and the accessory terminal.
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