[Misiur]

Hm, this might not be perfect, but let's use GetVehicleModelInfo.

pawn Code:

new 
    Float:X,
    Float:Y,
    Float:Z;
GetVehicleModelInfo(model, VEHICLE_MODEL_INFO_WHEELSREAR, X, Y, Z);
//IsPlayerInRangeOfPoint(playerid, 3.0, X, Y, Z)

Additionally you could add some offset using sin/cos functions, so only when player stands behind the car he'll can access the trunk