I'm continuing development from this question.
The schematic given by Jasen worked as a charm but little did I know that was with the connector plugged out, as soon as I plugged the connector in I found out that it no longer works. After a bunch of tries and failures I determined that while I'm essentially connecting a pair of pins, another one must be disconnected(?).
The connector consists of 4 pins in total which I've labeled P1 to P4 below, which correspond as shown to the previous question:
P1: Unlock P2: Common P3: - P4: Lock
Basically to unlock pins P1 and P2 need to be connected and P3 must be disconnected, or open. To lock pins P2 and P4 need to be connected and P1 needs to be open, where by open I mean there must be no connection between the plug and the socket.
This slightly complicates and adds on to the schematic from the previous question, here is what I came up with:
I see how it can probably be slightly confusing, labeled P1 to P4 are coming from the car and P1' to P4' go in the socket on the door, I've omitted P2' and P4' because they are directly connected to P2 and P4 (spliced connection).
I'm using an LM2902 as a logic not gate, since I already have it and I'm using only 1 channel of it so I decided I'll just use 2 more instead of buying a not gate. Basically the paths from P1 to P1' and P3 to P3' should be broken respectively as described in the scenarios above.
I also found out that the voltage between P2 and P3 (12V/0V) tells me the state of the lock but that's not relevant to the question - I'm just mentioning in case someone wonders.
Now there is definitely something wrong because only the lock part works and the BC556 that's connected to P1 and P2 immediately gets hot when it shouldn't especially as there is no signal on the unlock line to begin with. Cables around it got brown - that hot. After replacing it, that is. First one had decomposed with it's collector leg broken apart. I'm probably going to go ahead and use relays because I've spent way too much time on this problem, but I really wish to know what's wrong with my current plan because I just got excited about transistors..
PS: I apologize there are no labels on the components.
PS2: All 3.7k resistors were replaced with 3.3k resistors except the one on the state divider, but it probably doesn't really matter.