Electronic – arduino – Simplest way to switch inductive load with arduino

You really should post a schematic instead of all the hand waving. However, there is one obvious flaw in your description. One side of the relay could should be connected to the collector of the transistor as you say, but the other side must be to the power supply, not ground.

When the transistor is off, the collector is effectively not connected or left floating. When the transistor is turned on, it will pull the collector low. Since that end of the coil will be low, the other end must be high for there to be voltage accross the coil so that the relay can turn on.

Here is how you should hook things up:

Don't leave off D1. It is there to catch the inductive kickback from the coil when it is suddenly shut off. Without it, the coil will generate high voltage and fry the transistor.

All the methods will turn your car unsecure, do it with caution.

The easy way:

I think that the easy way is to connect the button in a input of the Arduino, then, Arduino send the command normally to the transistor. But its not the most reliable way, because if the Arduino for some reason doesn't work, your car won't work too.

The reliable way:

Is just put your button in parallel with the transistor, that will work as the example below.

^{simulate this circuit – Schematic created using CircuitLab}

The 3rd solution will work too, but you will have more work, because you will need get 5V somewhere to power it, and putting in parallel with the transistor is already there 12V.

Keep the diode the way it is. It is helping to raise the lifetime of the relay.