You would want to sum the outputs of your line converter if your factory system band passes the speaker outputs.
Meaning, the factory system has a dedicated output for highs, that go to JUST the tweeters, and a dedicated output for just mids to sent to the front door speakers, etc…
If your factory system outputs full range, then you probably don’t need to sum. Or maybe the system DOES band pass certain outputs to the front speakers, but also offers a full range signal for the rear speakers.
In these situations our line converters have a few different configurations scenarios that can address most issues.
You can find more information about summing here.