The 1999 Mercury Grand Marquis air suspension relay switch can be found on the firewall in the engine compartment. The air suspension relay switch will be on the passenger side of the firewall.
The relay for a 99 Grand Marquis heater is located on the kick panel on the driver's side of the car. The relay is located under the kick panel cover.
It should be located in the Trunk near the right side. If it's not there, it's probably located under the hood with the other relays. hth
In the Power Distribution Box in the engine compartment ( the P D BOX is " live " ) Relay # 303 ( one of the large relays ) is the air suspension relay
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the black box on the left side of the trunk
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Inside the front right and left fenders, just under the side marker light, I believe. If it doesn't work at all, check the relay first.
The relay is located in the engine compartment to the right of the heater hoses if you are looking at them. It dose not look like a regular rely. It has a connector that plugs into it.
Under hood, driver side, front side of wheelwell, mounted in relay block, BE SURE TO READ THE BACK OF THE LID
The air suspension relay on a 2003 navigator is located on the passenger side where the compressor is on the front bottom near the fog lights.
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You have many relays in that vehicle, most are in the fuse and relay box under the hood.