Driving a 12v LED, substituting a PWM chip for a linear regulator?
The circuit is more complex than just a dc-to-dc switching regulator.. You need to sketch out the circuit to find out how it works. The circuit board is simple enough it's a single-sided board that you can trace.

That said, the LED is a simple 3.3 Volt or so multiple amp package. With the right voltage and current limiting device, AKA a resistor, it should turn on if you solder directly to it. If it's not turning on then it's due to your power supply not giving enough current. That said, a switching regulator is more efficient than a linear regulator. If you're dropping 7 volts at 2 amps that's 14 watts of power it's wasting in heat.   Your regulator may be shutting off in thermal protection mode.