I've had my 6P since Dec. '15. It's never had good battery life (at best 4h of SoT). Now, I'm averaging 2 sometimes 3 hours of SoT. This is not a problem specific to your 6P. Just browse through the 6P subreddit or do a quick Google search. There are actually two battery problems: early shutdown (the 6P shuts down when there is <30% of battery remaining), and the poor battery health (leading to worse battery life/SoT). You can track the health of your battery by using an app called Accubattery.
You can try the following:
1) Install a new battery. Buy one from eBay or another seller, and install it yourself or have a shop do it. This is basically the only fix.
2) Try an RMA: Google has been known to RMA 6P's that were purchased from the Play Store (even outside the warranty period). If you bought it outside of the Play Store, you're basically screwed. You're not going to get anywhere with the Huawei RMA process.
3) You can try greenify and aggressive doze.
4) Full reset may help, but it probably will not.
As far as car mounts go, I found nothing works well. This partly has to do with the buttons being all on one side, and the clamp style mounts press on the buttons. I've also had magnet style vent mounts, but they don't work well if you take a turn fast, or go over bumps. In my new car, there's a spot to hide the phone away for Android Auto, so I just do that. If you find a good mount, let me know.
[Rant] The 6P is just a big clusterfuck, like typical Google fashion. We get screwed by the battery issues (both the early shutdown and the battery health issue), granted this is probably more Huawei than anything. I have lost faith and confidence in Google for Nexus/Pixel devices when it comes to software. We get the shaft when it comes to updates (Google Assistant took forever to roll out, and I still haven't received it), lack of consistency (why no Pixel launcher, or fingerprint gestures). All this while they promote the expensive Pixel. I bet the Pixel will get the shaft when they release the Pixel 2. I thought the 6P would be my first "two year" device, but nope. I can't wait for the S8/Note 8 and other flagships to come out.