2013 Corolla, power / accessory outlet works ........ BUT -

Thanks in advance for any comments, thoughts or advice here. My 2013 Corolla ( base model ) runs okay, 139,000 miles. Properly maintained, but still some problems and breakdowns in its history. But here's my QUESTION:

My accessory power outlet WORKS. If I plug in my cell phone, it WILL charge it up. Slowly, but adequately. I happen to be a rideshare driver and recently bought an outlet box that can allow up to 3 devices at once. As soon as I plug in a second device : the other device either will not work, or in the case of a windshield mounted camera, it'll shut off. There seems to be enough power to operate just the camera, if plugged in directly, and by itself. Or, just the cell phone. I tried with an old Garmin GPS. It also shuts down if I try two devices at once.

New battery, new alternator just 3 months ago - This is an old problem, not a recent thing, but I never needed to have 2 devices at once until now.

I have tried with 3 different types of "multi-outlet adaptors." Same result. It appears to me there is not adequate POWER to keep more than one device going at once. And since it works for one device I know it's not a blown fuse. As an Uber driver, I need to keep my cell phone on, plugged in - it's your line of communication to get passengers, directions etc. The camera is now something STRONGLY recommended - by Uber AND by the insurance company. ( Yes - I have ride-share insurance too ! )

Anyone else run into this ???

Do I need to have a second accessory outlet added into the vehicle ? With its own line to its own fuse ..... or ??? Agian, thanks for your thoughts and advice.
Sounds like you either have a short to voltage possible caused by your outlet box or you have resistance in the path wire to your accessory outlet. This resistance could be cause by rust on the wire, to the accessory outlet, or if there is other forms corrosion on the wire. Take a digital multimeter (DMM) and set it to voltage Test to see what the voltage reading that's coming from the accessory outlet. If you're getting less that 12 volts that's a good indication if corrosion on the wire leading to your accessory outlet.

Another test you can perform is to take a test-light and test your accessory outlet. If the test-light lights up bright, then that is an indication that you have good flow of power going to the outlet.