I Have a 1990 Lincoln mark vii, the headlights wont come on sometimes and i had to hook a wire from one of the headlights to the negative battery post to have control of them in the car with the light swith. Also the transmission wont shift out of 3rd gear and it dosent have Overdrive, someone told me about a shift cable or wire that might help if i replace it can someone help!

Have you called Lincoln-Mercury dealer? I had one go bad on a Grand Marquis LS and I never replaced it.
It can probably be wired around if a person is savvy on DC wiring. I know it cost a few hundred a decade ago. don-ohio

Here is a firing order diagram for that vehicle to help assist you.

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should imagine so ,standard size hole for it ,with the right wiring adaptor plug you should have no problem

All vehicles are equipped with a fuse panel located on the left side of the lower instrument panel.

I think this is the wiring diagram that you need for your 1990 Lincoln.

here is a wiring color break down.
1987 Lincoln Mark VII Car Radio Wire Guide
Car Radio Battery Constant 12v+ Wire: Green/Yellow
Car Radio Accessory Switched 12v+ Wire: Yellow/Black
Car Radio Ground Wire: Red
Car Radio Illumination Wire: Blue/Red
Car Stereo Dimmer Wire: N/A
Car Stereo Antenna Trigger Wire: Orange/Blue
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: 6 1/2′
Car Audio Front Speakers Location: Doors
Left Front Speaker Positive Wire (+): Orange/Green
Left Front Speaker Negative Wire (-): Black/White
Right Front Speaker Positive Wire (+): Pink/Blue
Right Front Speaker Negative Wire (-): Black/White
Car Audio Rear Speakers Size: 6′ x 9′
Car Audio Rear Speakers Location: Rear Deck
Left Rear Speaker Positive Wire (+): Pink/Green
Left Rear Speaker Negative Wire (-): Blue/Orange
Right Rear Speaker Positive Wire (+): Pink/Blue
Right Rear Speaker Negative Wire (-): Green/Orange

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the air lines or bags need to be replaced,check the lines at the pump

check the idle air bypass solenoid,take off and clean or spray out with carb cleaner.

Jumper battery power to the blower motor if it runs then replace the blower motor speed control resistor, it is located near the blower motor.