You need an RF Field Strength Meter that can read the magnetic and electric fields at X # of meters from the antenna and compare the output to another antenna install on the vehicle with a direct-coupled antenna. They are about $400 for a meter worth buying.
You also need an antenna and coax cable analyzer. They can measure cable and connection losses. Good one will even tell you exactly how electrically and mechanically long a cable is. Expect to spend about $550-$600 for a meter worth buying.
EDIT: Forgot to add, there is likely such small loss anywhere in that path (since its in a car and the runs are so short), that there is likely no room for any measurable improvement. Even if you make a measurable improvement, there will likely be no practical improvement that will be distinguishable by the operators on either end.
I can't speak about what your radio can or can't do, but Chirp is pretty straight forward. Use the image below as a reference for what the general meanings are.
LOC is the channel location, aka what memory slot.
Frequency is the simplex frequency of the channel you want to use. If it is a repeater frequency, you enter the repeater output frequency, which is what you will be listening to.
Name is just that. Pick a name that give you an idea of what channel you're on.
Now... the tone mode can be a little confusing for people. There are a few options to pic from, but until you get used to the radio and software, I would not set the tone mode for anything unless needed for a repeater.
The tone mode that is most commonly used is 'Tone', which your radio transmits sub-audible tones to open the squelch of the radio/repeater receiving your signal. If you look at my example below, I have Tone set on the repeater memories. But I also have TSQL on two repeater memories. That function transmits a tone to open the repeater and my squelch only opens when the repeater transmits that same tone. The reason I have that set up is because there are two repeaters near me, using the same frequency pairs, but they use different tones. So, by using TSQL, I know when I hear a signal, it is coming from one specific repeater.
So, you populate a value in the Tone column if you what to trip a repeater, but hear everyone. Use the TSQL column if you only want to here people or machines using a tone when they transmit.
(Note, there is a Cross mode. That is for radios that support transmitting one tone to open a repeater [for example], but having another tone open the squelch on their radio. I would suggest spending some significant time researching how that works before you mess with it.)
You will also notice that I have two memories configured for DTCS, at the very bottom. This mode is commonly called Digital Code Squelching. It is just another squelch type. Most repeater owners don't use this method. It is most common on simplex channels. It can also be insanely complex. You can have DTCS or Split DTCS, transmitting one code and your squelch opens with another code. Than you can reverse the code sequence on none, one, either or both. I would suggest spending some significant time researching how that works before you mess with it.
If you look at the image below, you will see which tone column is associated with what tone mode. That should help understand those. Just ignore DTCS RX Code, DTCS Pol, and Cross Mode until you learn about how those items work.
The duplex mode is going to be None for the GMRS and FRS channels, unless you are programming a GMRS repeater pair. Then it will be a Plus (+). If you are programming a repeater pair and select plus, the software should automatically select the proper frequency offset, based on the starting receive frequency. If it doesn't, you should enter 5 MHz (5.000000). This automatically changes your transmit frequency, when you press the PTT, to the repeater's input frequency.
For Mode, you can use FM for everything except the FRS channels 8 through 14. Those must be set to Narrow FM or NFM. Also, if you cannot program your transmitter to 0.5 watts or if your radio has a detachable antenna, per the law, you are not allowed to transmit on those frequencies. My radio doesn't go any lower than 1 watt and has a detachable/external antenna. So, I have them programed for receive purposes only.
marcspaz's post in Measuring losses was marked as the answer