Input filter for alarm

I'm looking into creating an external add on alarm for my van without an extra keyfob on the key ring, has it as been known for someone can open all doors with setting the factory fitted alarm off. The indicators flash twice when I double lock it, I think the flashes are 250/500Ms seconds apart and then when I unlock it gives a signal flash about 1 second long, my idea is to use and micro-controller to capture this signal to enable me to turn it of/on with having to keep remembering to turn it on. May I add an ultra sonic or infrared sensor along with 2 micro switch for the trigger if the rear doors open.
What I'm looking for is the best way to create an input filter from the indicators from 12v to 5v and give a clean switch signal, I've tried with an resistor divider with a low pass filter but with the length of the wires I seem to get a false spike which sets it off. At the moment I'm only recording and testing without any sounder so I can see what heppening.
Once I've managed to sort out the input filter I than can capture the full lengths of the flashes of the indicators.
I'm sure I seen some where using a schmitt trigger but not 100% sure how to do it
So any recommendations to achieve a clean switching signal from the indicators
Thanks

Where exactly is this 12V signal that you want to detect coming from?

The positive side of the indicator bulb.

After doing some searching I'd seen an article in nuts & volts magazine on input switching and filtering of real dirty input signals. The circuit attached uses an input filter R1,R2,C1 & C2, Then R3,R5 & Q1 act as switch then switching through schmitt trigger to make it a true clean signal output. The only reason I've used the NAND gate is so when both inputs of U1A & U1B high the output is low on U2A. The only reason I've done it this way is when the van is locked both indicators flash at the same time and to prevent false signaling turning the alarm on/off while indicating left or right.

Is this a bit over the top or can it be simplified ?
I need it to be reliable as possible as at the moment I'm only simulating it in ISIS before I build it to make sure it works or an easier method.