Do you live in a "rust-belt" state?
By "rust-belt," I mean a state where you live at least 8 months out of the year with salt being thrown down like there is no tomorrow, then the other four months, all the back (dirt) roads also have salt and calcium chloride dumped on them to keep the dust down.
If the majority of the underside is rusted and brown in color, and not the original color of the car, then you have rust issues.
I have found with more than one brand of vehicle that they end up with one or more grounding points becoming disconnected, rusted or corroded so badly that they no longer serve the purpose for which they were intended, that is, to negatively bond various (and numerous I might add)
, portions of the car.
When vehicles were all steel with steel bodies and steel frames, there were a lot less grounding points necessary than with these new cars that contain much plastic and other non-conductive materials securing all manner of electrical components and anything that needs electricity to operate, from pumps to motors to lights to all sensors, switches, etc.
When I looked in the shop manual, there appeared to be at least 20 or so grounding points and that was in the front alone (from the firewall forward).
Before spending money on "hit or miss" replacement of parts, I would get a good set of jumper cables and attach them as one long cable, secured to the negative battery pole. Use the grounded end of the cable to see if there is continuity between all your grounded components. Be careful about making any sparks where there may be some kind of flammable liquid nearby or on what you are testing, in other words, don't burn your car up by making sparks next to a gas leak.
You need to remove a lot of rust in order to get to sufficiently-bare metal to make a good reliable contact! A grinder works well as does a Dremel tool with the cutoff wheels when in tighter spots.