Vinyl siding issue......

Have a leak after getting new windows installed. Ive been able to narrow it down to what looks like the fact that wind-driven rain (we dont get the leak in a normal storm with no wind) is pushing water underneath the j-channel of our vinyl siding where it meets a roof line above our breezeway. This also happens to be right above the window that is leaking, so we have the intersection of a window, roof, and eave/overhang.
After looking further, I noticed that the j-channel used to finish off the siding where it meets the roof has no flashing behind it. So my guess is that the water is pushed under the j-channel then running down the roof but against the original clapboard siding, then running along the clapboard and into the house (the water comes in *behind* the new window).
Question is - where should I put the flashing? Should I remove the j-channel, add the flashing and then reinstall j-channel? Or can I bend flashing so that it kind of sits in the j channel behind the siding and then the rest covers the j-channel? Obviously (if you can understand my weak description) the easier fix is to rest it in the j-channel, but Im thinking the right way is to get it behind the only question at that point is should I pull the shingles off the roof and have the flashing underneath the shingles? Or just rest it on top of them (basically come out from underneath the j-channel).
Thanks! -Chris
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