An injection marinade is a thin solution that is placed into a meat injector. It is important to make sure that the marinade will pass through the needle. There are several brisket marinades that you can use or you can simply take some of the rub you used earlier and mix it with equal portions of oil, water, and vinegar. I suggest you use a good olive oil and cider vinegar. These flavors work very well here.
Load the marinade into your injector and inject it into the meaty portions of the brisket every inch. Use small amounts. Injection too much will simply cause it to leak out as the meat contracts from the heat.
It is best that you do this injection right before you place the beef brisket on the smoker. If you do it before you refrigerate the drop in the temperature of the meat will cause the marinade to push out overnight.
You can save the remaining injection marinade to use as a mop later on.