I posted some thoughts on it here
http://ecomodder.com/forum/showthrea...-etc-8684.html
The horn is likely not heat related, a capacitor across the horn terminals (the buzzy part) might help there.
I did make a couple revisions to the pcb recently (v1.3) which might help reliability namely:
the LED is powered via an 80 ohm resistor to 12v and an npn transistor. The resistor (orangish) does get hot so it is taking a lot of the load off the regulator.
This also allowed me to set it up so that the capacitors do not have to power the backlight in a brownout, so they should keep the cpu running for longer in a flakey power situation.
It also makes it easier to reloacate the resistor if you are using an enclosure, and move a large source of heat out of the enclosure, and/or to tie it into an existing dashboard dimmer circuit.
The other changes were mostly just making pads available for the unused cpu pins and establishing internal connections for avcc and agnd.