I saw a small 4-camera single dome unit and a 4-channel DVR system for vehicles at one of the video suppliers I used to purchase from. Looks fore and aft, port and starboard ...
Somewhere around $500., though
I guess you could mount a GoPro (or less expensive DVR-Cam) looking forward with a view of the rear view mirror to catch what's behind you for a lot less.
... or, a couple of these are less than $100. ($40.ea) ... even less on ebay
...
LINKClick to view attachmentHigh Resolution Windshield Mounted Dashboard Camera with DVR
Easy to install mobile camera is ideal for monitoring driver behavior
True HD video resolution: 1280 x 720 (720p) @ 30 fps
Integrated 2″ LCD screen
Built-in microphone (can be disabled)
SD card slot (supports up to 32GB of storage)
Motion detection
Flip down OSD provides convenient viewing and adjustment of video settings
Internal battery backup
120° max field of view
Mounts easily to windshield with suction cup mount