Rookie Questions on Gauge Installation

[RED92LX50]

Hi all, I am installing two Auto Meter gauges in a Dual Gauge plate with vents (where the middle air vents are on my 92). I have the Vacuum gauge, and the Volts gauge (looks like simple install for both, but I would like some clarification/pointers to help the install go smoothly).

Volts: The instructions say to route a ground wire through the firewall, and attach to a good engine ground. Do I really need to run this wire, or can I use a ground near my stereo, etc. If I do need to do this, what is a good engine ground (this does not mean to the Block does it - just solid metal (frame/body) contact in the engine bay?). What about running it directly to the Neg battery terminal? Also, Where is the easiest place to splice into the (+) Switched Ignition voltage for the Positive connection?

Vac: Will I need to splice a current Vac line and use the provided T fitting, or should I have an empty Vac port on the Tree?

For Both: Where is the easiest place to splice into the variable dash lighting for the gauge lights?

Sorry so long, I have been a board "Reader" for a long time, but have few Posts - thanks for your help!