If you take two speakers that are 8 ohm each and wire the together from the amp, both speaker positive terminals together and both negative terminals together, they are considered wired in parallel. You could accomplish this is a couple of different ways... one speaker wire to the first speaker, then from that speaker run another speaker wire to the next speaker... or you could run two speaker wires from the pos/neg terminals on the amp to each speaker. Either way you are connecting the two speakers together. By doing this with two 8 ohm speakers we cut the resistance in half to 4 ohms. So the amp sees a 4 ohm load instead of an 8 ohm load. If you have two 4 ohm speakers wired in parallel you would have a final load of 2 ohm. However, if you wired them in series, you double the resistance, two 4 ohm speakers would make a final load of 8 ohms.
View attachment 60378 View attachment 60379
If you are running factory power, one of the best inexpensive replacement speakers that I have used in several trucks is the CDT speakers...
https://www.cdtaudio.com/sep_components/subwoofers/cl69s.php
Before you replace speakers, consider treating the doors with dampening material... the outside skin which requires removal of the door panel and the inner door panel (videos on removal are on YouTube). Also add some closed cell foam between the door panel and the door to help eliminate rattles. This will typically enhance the factory speakers and in many cases make them acceptable, and/or better than some replacement speakers. I've seen it happen time and time again, someone replaces the door speakers and they lose the mid-bass because the replacement speakers are underpowered with the factory amp and/or not as sensitive as the factory speakers. Most guys don't bother to compare them by installing only one replacement speaker and then use the balance control to compare the two speakers. I suspect if they did, there would be more guys leaving their factory speakers connected and sending the replacements back.