Question is how much do you trust this guy? If it were me, I'd want a 2nd opinion unless he is REALLY trustworthy.

Can you remove the trunk lid carpet, take out the lock and remove the hose and trigger it remotely and see if it sends a vacuum to the lock?

If it gives you any idea, I am driving a 99 E300 and it has about 360K on the clock and the biggest expense has been the body work thanks to the WI salting of roads.

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow