What charger did you buy? Have you considered the possibility that you received a defective item? Amazon is full of cheap 12V USB plugs, but I suspect many of them aren’t worth the ridiculously low prices. One of the most popular brands is Anker and their stuff really isn’t that much more expensive than anything else. Of course you need to understand the power requirements of the USB devices you intend to plug in, as some chargers don’t support some higher power modes, but I would expect a radar detector or GPS would only need good quality standard 5V ports. If both devices need to be powered on simultaneously, you just need to confirm that the ports can deliver enough power (sometimes the second port will be limited depending on how much is being drawn from the first port). The 5V sockets are capped at 2.4A and units that can handle full power on both sockets are often advertised as 4.8A chargers.
