One of my favorite tips for cleaning dirty rubber floor mats (WeatherTech & the like) is to use UNDRESSED!
Given that Undressed Tire Prep was formulated specifically to clean and strip rubber tires- all while not causing further drying/browning, it’s a great option. The price point is also quite desirable at $16.40 a gallon!
Undressed is Ready To Use. No special dilutions needed, so just grab and go.
I spray the mats liberally, brush by hand or drill, and rinse! I do also clean rubber floor mats at the same time as I am cleaning the wheels and tires. Spray everything with desired cleaners, scrub everything, pressure wash everything. It reduces the number of times you’re picking up bottles and pressure wash hose and keeps you in your groove. Whether you’re an enthusiast or detailing as a profession, efficiency and time are money!