do you still have the factory stereo amp (not sub) or is it a full system amp?
if so, can you check the amp for possibly a low gain setting or poor connection on that channel?
IF so, check the connections to the speaker. Mine shook loose as follows.
did the speaker swap, adapter plates, harness adapter and all. The harness connector was bouncing free, unsecured and eventually pulled the passenger front speaker loose on one terminal. I went back and secured all of them and haven’t had that happen again.
If you’ve been amp’d ... Lots more potential troubleshooting to do.
first things first
using balance and fade, put an audio track on that uses the speaker, and see if there is sound, and if it’s full sound. bonus points: adjust the connectors while this is going on.
if you end up with low volume, you can try swapping the front speakers / optionally the harness adapters side to side and see if the problem remains on the same side or goes to the other side.
if it goes to the other side, it’s likely the speaker or connections. if the problem stays on the same side, it’s likely not the speaker.
good luck