With the GP relay disconnected and the ring terminals for that relay insulated (wrap a latex/nitrile glove around it under the hood), then under the dash:
- With the key off, if you pull the GP card under the dash and stick one of the multi-meter leads in the contact for the light blue wire, then touch the other lead to anything substantially metal - you should measure 0volts.
- With the key off, if you pull the GP card under the dash and stick one of the multi-meter leads in the contact for the light blue wire, then touch the other lead to anything substantially metal - you should measure infinite Ohms or "Open Circuit".
- With the key on, if you pull the GP card under the dash and stick one of the multi-meter leads in the contact for the light blue wire, then touch the other lead to anything substantially metal - you should measure 0volts.
If any of the above checks does not equal the expected outcome then there is something wrong with the blue wire circuit/wire/contact (GP relay trigger).
With the GP relay removed from the circuit, under the hood:
- Connect one test lead to the GP relay's red control wire (smaller red terminal), the other lead to something substantially metal (engine, frame, wiper motor base) - with the key off you should measure 0volts.
- Connect one test lead to the GP relay's red control wire (smaller red terminal), the other lead to something substantially metal (engine, frame, wiper motor base) - with the key on you should measure about 12volts.
If either of those above check does not match the expected outcome there is something wrong with the GP relay supply (the thin red wire circuit.
If both of those are working as expected, you should be clear to install the new GP relay you pick up to the factory wiring.
In general it is safe to ground the blue wire to turn on the glow plugs, when unmodified factory wiring is in place (another reason we advocate here for not going cuckoo with changes from OEM). The GP card has an electronic switch (called a transistor) then when active connects the blue wire to the GP card's ground wire. Doing many switches in parallel from the blue wire to ground would have the same effect - whoever is connecting the circuit to ground is in control.
This is why pre-wiring a glow-plug switch as a backup is safe to do with a functioning GP card.