It isn't as simple as that Eshal. The constant current idea is good except that you need the current to be constant at the level of full brightness and the problem at the moment is the current is too low unless your engine is at high revs. Constant current works well when you have extra current in reserve and just want to set a maximum amount you want to flow, when there isn't enough current to start with, there is no way to boost it to a higher level.
What you might be able to do is, if your lamps are 12V, change them to 6V and use either a 6V battery with a circuit charging from the 12V alternator or you could use a voltage converter to stabilize the variable alternator output voltage to 6V at constant current. That would give constant brightness regardless of engine speed but there is no way to produce extra power from an already maxed out limited resource. An alternative would be to replace the light with a more efficient type, conventional incandescent lamps are cheap to buy but very inefficient, they produce more heat than light. LED would work and be far brighter than your present lamps but also much more expensive, even if you build your own. With todays technology you are probably looking at needing an array of perhaps 20 or more high intensity LEDs to give enough light output.
No easy solution I'm afraid :-x
Brian.