cbarge
World Famous Barge in a Budget
Typically,anywhere for the ease of installation and later replacement..cbarge, where you mounted your control box?
Basically lay out the wires before hard wiring everything to see where it will go.
Firewall is preferred but A/C cars tend to have busy firewalls.
Some people put the ECU on the driver side inner fender near the ballast resistor.
All a matter of personal preference.
Make sure you have it well grounded.