Just having one negative trigger button that can handle car volts works for the door unlock or car alarm aux output as well as a normal button. Did that on mine, but I like knowing what triggered it too. The heartbeat is useful, but should be enabled by the software just in case you have to reinstall windows or something and the software side can't be run.
I could tell you a lot about USB and standby. The computer may or may not keep USB power on depending on how it's configured so using that to determine if it's on can be a problem. Oh it also helps to have something to turn the monitor on/off, amp remote like you have and external USB power or other POL.
Well, just a little update, I'm still quite active with this project, just haven't had much 'eye candy' to share, as I've been working entirely on the software/firmware side of things. Multi-threaded C# software communicating through a C++ DLL via USB to the SDC board, which runs firmware in C and (among many other things) communicates via I2C to the auxiliary board, which runs firmware in C (using a different C compiler, naturally), and handles the LCD. As you might imagine, it's been a headache, to say the least. :tard:
But, things are coming along very well - I'll probably post up a screenshot of the current software soon, once I clean things up a bit.
I'm definitely looking forward to diving back into the more fun and 'glamorous' hardware side of things, and will probably be throwing the main SDC board onto a PCB before long.