I bought the Eberspacher Espar Hydronic S3 D5E Diesel Heater Kit and had a question about the on/off switch. Is this meant to be used instead of the EasyStart Pro controller, or should both be installed?

The primary controller supplied with this Eberspacher Hydronic S3 D5E diesel heater set is the EasyStart PRO. The manual on/off switch installation in addition is optional, it is not necessary to install this switch for correct functioning of your heater.  

• The EasyStart Pro controller is the optimum controller for Eberspacher Hydronic S3 D5E diesel heaters, offering timer functions, temperature regulation, altitude compensation, and diagnostics via CAN‑bus. It’s designed for permanent installation in the cockpit or dash panel.
• The provided on/off switch provides a secondary manual operation option, but it does not replace the EasyStart Pro or enable any smart control features.

Install both as an option:

• EasyStart Pro = your main control interface
• On/off switch = manual operation as addition if wished