Small Suggestion. If you use the Door sensor and wish to trip it directly from 24V, try putting a transistor (NPN) across the switch contacts. (Collertor on Positive side of contacts - Measure if unsure.) You can common the minus side with the grouond of the 24V system because the battery power isolates the Door switch. Use a resistor from the 24V you wish to sense to the base of the transistor large enough to send an mA or so into the transistor. IF THE Off voltage of the 24V is greater than 1 Volt or so, feed it through as many diodes as needed (.6 V per diode) to eliminate that. Should be cheaper than a relay.