This is what I added to mine. Clock and indoor/outdoor thermometer as well as a voltmeter that monitors chassis battery.
I put the outdoor thermometer in front of the radiator on the cab. I wanted to monitor air temperature while driving, especially when temps might be approaching freezing. Works great while moving. Obviously when parked it falsely reads temp including built up heat under the hood. Clock works great. Lights up a light blue at night. Has battery built-in that maintains time when ignition is off.
I removed the cigarette lighter plug and wired direct to a power source that is on when ignition is on.