Why doesn’t my Plymouth Duster have a passenger side mirror?

Replace your small A body rear view with a larger B or C body one.
It makes a big difference.
Use that and the driver's door mirror to keep track of everyone around you at all times..
Always watch your rear especially when starting to brake.

Always glance over either shoulder before changing lanes.
It's to make double sure.
(This has saved me numerous times when someone got up beside me into my almost blind spot around the C pillar or came up fast)

On a side note, many times when someone is tailgating me, I reach up and touch my rear view.
Maybe even adjust it.
If noticed that many times they will back off because they know I'm watching them.

I don't think a passenger door mirror is much help with these tips.