[Cxnnor]

Took out part of the callback and added

pawn Code:

if(!IsPlayerInAnyVehicle(playerid) == 1)){
        DestroyVehicleForPlayer(playerid);

If you get compile errors tell me and I will attempt to fix them.

pawn Code:

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if(newkeys == KEY_SECONDARY_ATTACK ){
        if(!IsPlayerInAnyVehicle(playerid) == 1)){
        DestroyVehicleForPlayer(playerid);
            new Float:x, Float:y, Float:z, vehicle;
            GetPlayerPos(playerid, x, y, z );
            GetVehicleWithinDistance(playerid, x, y, z, 20.0, vehicle);
            if(IsVehicleRc(vehicle)){
                PutPlayerInVehicle(playerid, vehicle, 0);
            }
        }