Quick question about the best way to loosen bulb / socket that has become fused by corrosion. After disconnecting and unmounting the taillight, I have thought of a baking soda / water solution, liquid wrench, soap, etc. Repeatedly applied with a brush then wiggling until loose after which a full treatment with steel wool, wire brush etc. What may be the best treatment for getting these loose?

 

PB blaster might work.
Baking soda mixed with vinegar might be worth trying too. Both are highly under rated and cheap.

 

Just soaking in plain vinegar is best because rust is alkaline (oxidized metal). Vinegar would dissolve it. I use exactly that for zipper pulls oxidized by saltwater, as well as badly-oxidized brass.
Alkaline baking soda combined with acidic vinegar turns into salt, not as aggressive against the alkaline rust, and also cause more rust.

 

Pickling/cleaning vinegar is higher acidic content. 7%. Rust will fall right off anything soaked in vinegar. Do not soak CAST in vinegar..it will get eaten! Use silicone grease on bulbs/ socket to prevent corrosion.
I’ve done 3 frame off restoration of vintage autos and a bucket of vinegar always on hand for soaking small parts.