I hope this helps clear up a few questions found here and on QRZ about mobile installs. I see conflicting advice, and a lot of it is dangerous and lacks fundamentals. The two most important aspects of the installation are Power termination points and Cable routing. Although well-intentioned, many hams and even radio manufacturers recommend outdated and hazardous installation practices concerning the radio's power wiring. Both fail to look at the circuit they create when they recommend connecting the radio's negative power wire directly to the vehicle's battery negative return post.
The first drawing clearly shows the circuit. You place your radio in a dangerous ground loop. Note the "Battery Negative Chassis Return Wire." Pop your hood, and you will see it. That wire carries all the vehicle current back to the battery negative terminal from the chassis. When you connect the radio's battery negative directly to the battery, your radio is in parallel. It is a piece of wire hopelessly lost in a ground loop. Your radio has a vehicle current flowing through it. It is undeniable.
What makes it dangerous is that if something happened to the vehicle's bonding jumper, you would not know it until you tried to start the vehicle. Place the key in the ignition, and all the bells and whistles work because your radio is now the bonding jumper. Turn the key, and the cab fills with smoke with engine cranking current flowing through your coax and radio negative wire.
It is dangerous and a root cause of many RFI issues. You are forcing vehicle current through your ground plane, aka Common-Mode noise. Popps and clicks, alternator whine, ignition noise. It is all flowing through your radio. Try an experiment if you have a clamp-on amp meter. Clamp the radio negative, turn the radio off, monitor the amp-meter, crank the engine, turn on accessories, and watch the current flow.

The solution is simple, terminate the negative radio wire to the chassis. The radio positive goes directly to the battery, not the negative. Where do you terminate the negative to the chassis? Due to the negative being both power and ground, you keep it short as possible, measured in inches. You do not want to drag it under the hood, and there is no reason to in modern vehicles. All new vehicles have electrical hard points under the seats and firewall. For example, all have high-power accessories under the seats, like sub-woofer amps, high-powered audio amps, electric seats, heated seats, and Inverters. Find one of the plugs and locate the ground wire; it will go to a hard point near a seat bolt for high-current connections. Find the green wires going to the vehicle chassis
For best Mobile Installation Practices, I recommend you use what the pros use. To do mobile electronic installation work for public safety, first responders, the military, any government agency, or utility, you must be certified and follow "Motorola R56 Appendix G Mobile Installation Standards and Techniques".
Standards and Guidelines for Communication Sites
https://www.sno911.org/images/Documents/WirelessTech/RRP/Policies/R56_Mobile_Installation_Standards_and_Techniques_2.pdfG.10 Power Wiring warns the installer of the dangers of terminating anything directly to the battery negative term post and never using a fuse on a negative wire. It contains excellent information on anything you want about mobile radio installs, right down to tools and materials. It even tells you how to install radios on motorcycles and how to mount antennas on fiberglass.
So next time someone tells you to connect your negative radio wire directly to the battery, say no, thanks Sammy, not in my vehicle.