No you can't tell wattage from teh remnants. And it sounds like it wasn't wired effectively.
You describes lights wired in series. That is not an acceptable way. Could start a fire or you will not get enough light out for the wattage they were. One was likely a bit brighter than the other.
Anyway KC and some other companies make a nice back up light that you could wire into your back up lights. No matter what you do I hope you use a relay with separate power for the lights in parallel. You could use the existing back up lights as the switch power to the relay.
I have the KC version for 5 years and couldn't be happier with the amount of light. I placed the mounting bracket between the rear bumper and frame. The lights are above the bumper pointed down where I needed it.
Hope this helps.
"Never look down on anyone unless your helping them up"- Jesse Jackson