Hi Anubi 5,
When your own hair on your sideburns grew out the darker roots came out. And that caused you to have to re-bleach each week. You have a couple of options. You could just shave off your sideburns and we could build them into the full cap and make everything the same color. Or you could keep your sideburns and blend them into the blonde full cap. Without seeing your hair style it's difficult to advise you on how this would look but I can tell you that it's definitely possible. It's not uncommon for sideburns to be a different color--especially as a guy grows older---his sideburns and temples usually go gray before the rest of his hair.
Regards,
John