You have lossy battery connection. either +, or -, or both.

To find it, you need to measure the volts at the lighter terminals
relative to the battery.

Here's one way:

*get some jumper cables on the battery
*then measure the volts from the lighter + terminal to the jumper - terminal
see if its battery volts (12.6 with the car off)
*measure the volts from the lighter - terminal to the jumper +
see if it's 12.6V

Likely, one of them will be low and then you have an idea if its a bad battery/+ connection or a bad ground/- connection.

The good news is that when you find the ratty ground or battery/+ connection,
and fix it, you might head off a bunch of way more serious issues.

Bill