Any way to upgrade the power output of the USB plug?

I've seen addons that you can put into that slot that output a combined 3.6A, 2.4/1.2 ea port. Here's a link to one:
creeronlineshop | Rakuten Global Market: YAC ★ YAC SY-A9 Aqua dedicated USB port 2
I'm not sure if it maintains the audio radio hookup, but this one has what looks like a usb passthrough. I assume to bump up the amperage and maintain connection to the headunit:
creeronlineshop | Rakuten Global Market: YAC ★ YAC SY-A5 Aqua-only USB port
None of these are necessarily the best price, I just grabbed the link so you know what to look for if interested.

Just in case you don't care about the usb connection to your headunit and don't want to splice/solder or tap, here is IMO the simplest solution:
Scosche USBC242M 12 Watts per Port 24W 4 8A Total Output USB Car Charger The | eBay

 

[QUOTE="Any way to modify the car and pump up the power output to that plug?"[/QUOTE]

That port outputs 500mA limited by USB 2.0 protocol (3.0 supports up to 900mA) so you can't draw more than what the head unit puts out. It's like telling the McDonalds worker to put in more fries, then crying when the extra fries just spills over the top Get an Amazon basics 4.0 Amp 20 Watt USB charger and a good USB cable (some USB cables don't draw the full 2A). Your cigarette lighter socket puts out like 10A but there's a reason there aren't any 10A chargers... Yet!

 

I use the USB plug for a flash drive that holds my music, so I plug other stuff into the cigarette lighter using an adapter like the one pictured below that turns it into one or two USB outlets. These adapters can put out up to 2A or more of power, depending on which one you get. Just Google "cigarette to usb adaptor", there are a ton of options out there.

 

Check Monoprice. They have all sorts of car chargers at good prices.

 

Multi plug USB charger available in market which can connect all phone models.

 

I got one of these from Amazon..have not tried it yet

 

I have had a similar issue in the past but instead of the USB not charging my phone, I wasn't able to use the adapters to charge and would HAVE to have it plugged in to the USB. I just picked this up:

And its the first one that has worked. Something like this may help with your GPS since its a bit more powerful than your regular car charger. ( Note: The link provided is for a Type C USB. Most phones are still Micro-B )