Self cancelling indicator project. Help needed
A partial answer, in outline:


Yes, a 555 will work.  I'm not thrilled with using a 555 for a 5-second interval because caps that large leak, but such is life.  Be careful shopping for caps -- you need something in the automotive temperature range, with low enough leakage that the time constant won't be significantly affected.
I would get a 5V linear regulator, but one that's designed for automotive.  Just follow the data sheet.
Assuming that each of the three inputs (left, right, brake) is +12V active, I would use a transistor buffer on each input, as shown.  The idea is that you need something that'll be reliable down to around 6V (for that bad day when your charging system fails you), but will survive a load dump (Google for it) and protect the rest of your circuit.  Double-check that a 1N4148 is sufficient for a negative-going load dump, and then decide if you need something that'll resist more voltage.
Put a bog-standard relay driver on the output of the 555; don't forget the flyback protection diode, and use a transistor that'll handle the load dump.  Most 555's will drive a pretty healthy current, so you can probably use an NPN, but use a logic-level MOSFET at your discretion.








simulate this circuit &ndash; Schematic created using CircuitLab

Some reasoning:


A 2.2k\$\Omega\$ pullup is a bit firm, but isn't a bad value.  It should demand around 2.3mA from the transistor collector in saturation.
For not-super-beta small-signal transistors you want about a 10:1 collector to base current ratio in saturation.  So, about 230\$\mu\$A into the base.  33k\$\Omega\$ flows about 310\$\mu\$A when the base is at 0.7V; 70\$\mu\$A of that is bled off to ground by the 10k\$\Omega\$ resistor.  The drive is a bit wimpy at 6V supply, so you can consider adjusting the resistance downward if you feel the need.
The diode protects the transistor B-E junction from reverse voltage transients.  Putting it on the low-voltage side of the input resistor should protect it well.
Even if the circuit sees a 100V transient in either direction (which is quite unlikely), that's only 3mA in either the diode or the transistor base, so you should be OK there.
The 10k\$\Omega\$ resistor is there to make sure the transistor turns off -- in case you're wondering.