Here is exactly what the installation guide for my Sony head unit says:
"When your car has a built-in FM/AM antennain the rear/side glass see 'Notes on the control and power supply leads.'"
Notes on the power supply lead (yellow):
- When connecting this unit in combination with other car stereo components, the connected car circuit's rating must be higher than the sum of each compnent's fuse (?)
- When no car circuits are rated high enough, connect the unit directlyto the battery.