If you don't want to spend the $140+ for the auto eject you could opt for a warning light on the dash. Radio Shack has some neon lights that mount on a round hole, that can be directly connected to 120VAC for about $3. Neon lights last a pretty long time, so it won't hurt for it to be on all the time. You could even add a 12V relay that cuts power to the light when the truck is off (hooked through normally open contacts). This could all be done for less than $20. Beepers, bells, and or whistles could also be substituted or added to the light circuit (assuming the use of the relay so it's not on all the time). If the beeper is 120V just tap it in, if it's 12v use a relay with a 120V coil to switch it.
You would have to splice into the wire going to the heater with a weathertight box (surface mount, about $3-4 at Lowes or Home Depot), and most likely mount a weathertight male motor base plug to it. That would allow a standard drop cord to be used. I would get the drop cord with the pilot light built into it. They make compression fittings that will seal around a standard size round cord for routing into the weathertite box to the block heater and indicator light.
Keep in mind what was said earlier in this thread by CstlEng about salt when locating and sealing all connections. Also route any 120v carefully so it won't get the insulation rubbed off, and cause it to become an arc welder the next time you plug in.
Of course this will only work if there is still power on the drop cord-if you use a timer that's off, or a GFI has tripped, no light.
Link to male motor base plug (found with Google search, know nothing about vendor)
http://www.mobile-mart.com/motor_base_plug.htm
Other thread about block heaters
https://www.turbodieselregister.com/forums/showthread.php?threadid=87239&highlight=block+heater
Link to Radio Shack panel mount lamps:
http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_011_004_005_000&Page=1