take encaps off, look though each header, the one you can look through gets the caps (where the water makes a u turn), the one you can't look through gets the input/output connectors.

If you do connect more than 2 units, it is beneficial to remove the baffles (some units allow for that) in the input/output ends of all, install the input connector there and a cap, install the other cap at the far end, with the output connector, feed into the next etc.