We have a 2000 Suburban that has 80k miles. It's sat a lot of its 18 years. Has mainly been used for long trips/vacations. Preface: we've found out with our maintenance that the early 2000 Suburban models have a combo of leftover 1999 and new 2000 parts!

The rear air unit & upper brake light went out last summer. Mechanic replaced the rear air unit twice (along with fuse & bulb in light -- per his word). We paid a lot of money to get the unit working and still nothing. He says that the wires have corrosion from spending time in salt air (based on consulting with a friend). He did not find the corrosion - he just says it is corrosion based on consulting with someone he knows. He was not an expert with electrical problems and did not want to go further with the issue & charge us more money (appreciated). While the vehicle did make several trips to FL over 17 years, it was not a permanent resident there. Mechanic has questionable business ethics based on feedback from many others.

The brake and tow lights work perfectly fine -- though they are probably wired separately.

We are not electricians so looking for guidance... Could it be corrosion in the wiring going back to the unit? I know it could possibly be a break? The rear air unit itself?

Additional issues: Over the past two weeks the clock has started to reset back to 12:00 and the horn does not blow. Battery connections have been checked for corrosion and appear to be clean.

Any assistance/advice is greatly appreciated.