Well several of those already have controllers wired into the circuit, which need
disconnecting alas - alternatively you can figure out the switch wiring that goes to
each controller (several flashing modes, presumably), or you can break-out at the
-ve battery terminal - again cutting traces needed
I'd sort of forgotten that LED bikelights tend to have flashing modes (non of
which are technically legal in the UK except for turn indicators).
If you're prepared to irreversibly modify them, just treat as a battery and LED,
break out at battery -ve. If you can fit a logic-level n-MOSFET in the case then its
source -> battery -ve,
drain -> LED cathode.
bring out source and gate as a twisted pair or screened cable back to arduino.
At arduino end gate via 150 ohm resistor to a digital/PWM pin, source to ground.
Repeat for each light.