You should try to obtain a lower level bill of materials (BOM) for the control box and work the reliability estimate from estimating the failure rates of the lower level components contained in the box. Tools for doing this include MIL-HDBK-217F, Notice 2 and the Reliability Information Analysis Centers “217Plus” reliability prediction procedure. The overall control box failure rate is estimated by summing the failure rates of the lower level components.

Another quick “back of the envelop” type estimate can be obtained from the Quanterion Automated Reliability Toolkit (QuART), “Reliability Potential” tool, which makes an estimate of MTBF based on an estimate of part quantity, using MIL-HDBK-217 relationships. See Figure 3 at the following link for more details on this tool (a free version is available for download):