Is the 1988 twin-cab 4x4 a 12v negative earth ?

Depends if it is positive or negative earth.
If negative earth, the positive post. If positive earth, negative post.

Clean battery connections and lightly coat with Vaseline. Check the earth lead on the negative connections has a nice clean bond to the bodywork.

There doesn't appear to be a main fuse, but check on the positive battery lead.

https://knigaproavto.ru/shemy/en/pontiac/firebird/850-1988-1992-chevrolet-camaro-and-pontiac-firebird-fuse-box-diagram.html

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Cars have been negative earth since about 1964

Have a look inside your engine bay .. the black battery cable goes to the bodywork/engine block ...? That's negative earth

Are the glow plugs single or dual wire if they are twin or dual wire one wire is positive and one is negative. I will base this on twin wire
I hope it is a negative earth system as I won't bother with positive earth instructions at this stage.
Get a multi-meter that has alligator clips on the cables attach black to a good ground then with the solenoid power wire removed on the starter (so it wont crank) get someone to hold the ignition switch in the on position, then test each pair of wires one will show voltage this is the positive one (mark each positive wire) now release the starter switch and change the multi-meter to ohms test the wires that did not show as active they should show good low resistance (hopefully 0) connection. now you can reinstall the glow plugs making sure positive on the glow plugs is paired with active positive wires that were marked then attach ground wires to the glow plugs.
Reattach the solenoid power wire and try starting it

What unit was fitted to the vehicle. I have an 1988 RD colt that had the factory AM/FM radio. This had a 6 pin connector and it works out easier to use the power and earth of the original connector and install new speaker wiring for the following reason. The AM/FM radio didn't need specific polarity for the speakers so it used 4 positive wires to the speakers the negative terminal of the speaker was then earthed to the body. This set up is not suitable for modern head units and will damage the integrated circuits. Therefore you will have to run new wiring to the speakers. You will also need to run a new constant power supply as the colt didn't come with this. Hope this helps

it doesnt matter as they work either way around ,the speaker isnt earthed

There is a loose electrical connection in the dash board display . Usually it is the earth point ... Negative or a ground

The negative cable from battery must go to body/earth
If you connect to engine you will have no decent earth for electrics as engine has rubber mounts
Is your corroded terminal causing problems,if not pour boiling water over,let cool,remove and clean.apply terminal grease

e36 m3 cab flat battery.i have now fitted a new 1 and my alarm will not work and the car is immobilised.the key fob is black with 1 button in the centre and the car is a 1995 can you help.

try having the air flow meter just behind the filter checked i hav a 94 proceed had similar problem and the air flow meter was the cause