Video input to 2010 OEM JBL w/nav?

Does anyone know if there's a way to get video input displayed on the screen constantly, even while moving, but without spending hundreds of dollars on adapters? I know I'd have to use the built-in video input for the back-up camera, but I haven't seen anything inexpensive that would just turn that input on constantly, or even better, be able to switch it on and off manually.

I'd like to attach a computer to the OBD2 port and have the readout on the display, but once the price goes much over $200 or so, it starts making economic sense to just buy a new headunit/computer with those functions built-in, or just bypass the in-dash system entirely.

I'd welcome any thoughts on this. It'd be great If I could have just one system to do everything - right now it looks like I need one display for car information (probably ultragauge), 1 display for improved navigation (Garmin nuvi), my cell phone for phone calls, and keep the OEM display for audio. I like the idea of having one other display, but this is starting to get cluttered - where do I put all this stuff?