The ECU is watertight and can be mounted just about anywhere so long as it’s away from “noisy” components such as coils, ignition boxes, or distributors. It could’ve been put under the dash, but it would have required a fairly large hole in the firewall that would need a cover plate. Instead, we chose a good spot on the inner fender that allowed hiding the harness and relays behind the brake booster. Harness clearance gets tight near the throttle linkage, so we took care to fasten it neatly in this area. The instructions were very explicit about connecting the ECU power leads directly to the battery. To do this, we had to extend the leads a little, but it was worth the trouble. Any stray electrical pulses or surges will affect engine idle and smoothness. Connecting directly to the battery avoids this.