Finding a schematic will be very difficult (if you can find one at all). Uniden would be very unlikely to send you one. Most manufacturers nowadays refuse to give out schematics and/or service manuals to consumers (many of them wont even give/sell them to service repair companies either). There are a few companies/sites that may sell you a copy of the schematic if they have it available.
As for the connector pinouts; the 3-pin power connector is pretty self explanitory (just match the pins up to the 3-wire power harness. RED = ~12vdc/13.8vdc, ORANGE = dimmer/illumination, BLACK = ground). As far as I know the serial connector uses the same pinout as a computer DB9 serial connector (they do this to allow direct connection of a serial GPS without the need for an adapter), although there may be a few unused pins.
What are you trying to do that requires a schematic?