You did not specify exactly which Jensen amp you have. None of the Jensen PowerPlus series I found had high-level inputs. You can go here
and select your amp to download the manual. They should also have the connector for speaker inputs.
Most amps that have both high-level and RCA inputs allow you to use one or the other, but not both at the same time. If your head unit has RCA ourputs, those would be the ones to use, because they generally provide a cleaner signal to the amp with less noise.
Hope this helps.